Isolation-by-Distance and Outbreeding Depression Are Sufficient to Drive Parapatric Speciation in the Absence of Environmental Influences
Figure 5
False-Color Depiction of Genetic Clustering on the World Grid.
Dark blue represents unoccupied cells or cells from which genomes were sampled that were not connected to any cluster. Each other color represents a set of gametes with genome sequences that are identical at more than 40% of their nucleotide sites, created according to the algorithm described in the text. Gametes colored differently have genomes that are identical at 40% or less of their nucleotide sites. Using this threshold in combination with θ = 0.6 helps identify different species with different colors. Note that (1) colors were assigned anew in each plot, so particular colors do not track the same lineage across plots, (2) the clustering algorithm is probabilistic because it is computationally expensive to compare every individual with every other individual for determination of genetic difference, and (3) some clusters are too small to discern in these plots. Plots (A1) through (A12) depict snapshots of the clustering state at 12 different generations during the simulation run with low movement distance of δ = 1.5 and outbreeding depression threshold θ = 0.6, the same simulation shown in the (A) portion of earlier figures. Also following previous figures, plots (B1) and (B2) were generated under a low movement distance of δ = 1.5 without outbreeding depression. This model shows no evidence of clustering at the difference threshold of 0.6 for two representative generations (250,000 and 500,000), nor for any other generations examined (not shown here). The (C) plots, for a simulation run with a larger movement distance of δ = 5 and outbreeding depression enabled at a threshold of θ = 0.6, presents a similar lack of clustering.